After drilling, a borehole, a casing or a liner is run into the well by submerging the assembled string of a casing. Occasionally while doing so, the casing gets stuck due to a local collapse of the borehole around the casing, and the casing can consequently not be submerged any further. In order to locate the area of the collapse, a logging tool is submerged into the casing. When the area of the collapse is found, a perforation gun is run in to perforate in that area to loosen the casing. If this is not possible, the casing is cut just above the collapsed area.
The casing may be cut by explosives, which is dangerous, and there is therefore a need for a mechanical solution for separating the upper casing from the lower casing without getting stuck.